The present invention relates generally to regenerator energy exchange units, and more particularly to the heat- and humidity-adsorbing matrix through which intake and exhaust air streams flow.
Regenerator energy exchange units, and in particular energy recovery wheels, are commonly used in building ventilation systems to precondition outside air that is to be introduced into a building so as to maintain a healthy indoor air quality. Depending on the climate and the season, outside air entering a conditioned building is typically either hotter or cooler, and either moister or drier, than the inside air. This invention primarily addresses the high moisture transfer rates required under humid outdoor summer conditions with hotter, more humid outdoor air. The outdoor air thus presents a load to the building""s heating, ventilation and air conditioning (HVAC) equipment in the building, because the difference in temperature and humidity of the outside and inside air must be accommodated by that equipment. Inside (conditioned) air is typically exhausted to the outside to accommodate the introduction of fresh outside air. The conditioned inside air being exhausted can thus be used to precondition outside fresh air by transferring heat and humidity between it and the entering fresh outdoor air. The outside air can hence enter a conditioned space which is nearer in temperature and humidity to the desired inside conditions, and therefore the outside air presents a smaller load to the building""s HVAC system. The desired comfort zone for indoor air conditions is between 40 and 50% relative humidity (RH) in summer conditions.
Rotary energy exchange devices typically contain a matrix made of a heat-exchange material which may be coated with a desiccant to provide for the transfer of moisture. Such devices operate by alternately storing and releasing heat and humidity. The matrix material is typically a porous or channeled structure that allows air to pass through it. The rotary exchange wheel is placed in an airflow path so that an intake or supply airstream flows through one half of the wheel in one direction and an exhaust airstream flows through the other half of the wheel in the opposite direction. The opposing airstreams are separated by a seal, and the wheel is slowly and continuously rotated as the airstreams pass through it. Because of the heat capacity of the matrix material, heat is absorbed from the warm fresh intake air and is then released to cooler, drier exhaust air. Because of the presence of a desiccant on the matrix, moisture is adsorbed from the warm fresh humid intake air and is then released to the cooler, drier exhaust air. Thus, the intake air from the outside, which is required to maintain a healthy indoor environment, enters the conditioned space cooler and drier than if it were introduced directly from outside, hence imposing a reduced load on the HVAC system.
Outdoor temperature and humidity conditions during the cooling season often cover a range from high temperatures with moderate relative humidity (e.g., 95-100xc2x0 F. with a RH of 40%-50%) to moderate temperatures with high relative humidity (e.g., 75-80xc2x0 F. with 90%-100% RH). Both of these conditions represent high absolute humidity levels (i.e., high dew points of 72-78xc2x0 F.) in which a large amount of moisture must be removed from outdoor air to reduce the humidity to indoor comfort levels. An effective energy recovery wheel must therefore be capable of efficient operation over this range of conditions, i.e., it must exchange moisture effectively between airstreams having relative humidity values between 40% and 100%. This requires selection of desiccants which operate effectively over this range.
A desiccant functions by adsorbing moisture from its surroundings into micropores of the desiccant material. The capacity of a desiccant for moisture adsorption is a function of the type and amount of desiccant used. However, desiccants vary in their inherent adsorption characteristics, and the use of large quantities of a desiccant having a relatively low adsorption loading curve will not necessarily result in greater moisture transfer. Therefore, materials having a greater capacity to adsorb moisture can be used in smaller quantities, and this feature can provide a significant economic and practical benefit.
The equilibrium moisture adsorption performance of most desiccants can be described by a simple loading curve of weight percent water adsorbed versus relative humidity of the air in equilibrium with the desiccant. FIG. 1 shows typical loading curves for three different classes of desiccant. Molecular sieves, also referred to as zeolites, adsorb water most effectively at low relative humidities and are commonly used for drying substances to very low humidity levels. However, zeolites have very little additional adsorption capacity in air having a relative humidity above about 30%. Desiccants with very convex adsorption curves, such as molecular sieves, are often referred to as Type 1 desiccants in the Brunauer classification system.
Normal silica gel is a very popular desiccant for energy recovery. It has a relatively linear adsorption profile and will adsorb moisture comparably at virtually all relative humidity levels and will reach a maximum capacity of about 40% of its weight. Zeolites, by comparison, tend to have a lower maximum moisture adsorption capacity. Linear desiccants such as normal silica gel are often referred to as Type 3 desiccants in the Brunauer classification system. Often the normal silica gel characteristic begins to flatten out at high relative humidities approaching 100% as shown by the line curve in FIG. 1. This reduces their effectiveness somewhat at high relative humidities.
It is an object of this invention to provide a means to enhance the moisture transfer capability of rotary energy recovery devices at high relative humidity conditions by employing a different silica gel, shown also in the loading chart of FIG. 1. This so-called xe2x80x9cmodifiedxe2x80x9d silica gel has a profile which is concave at low relative humidities with a steeper slope at high relative humidities. It also has a considerably higher total moisture capacity than either zeolites or normal silica gels. It may be referred to as a Type 5 characteristic in the Brunauer classification system. Such desiccants have largely been ignored by designers and engineers who are typically seeking deep drying characteristics, and the best drying activity at low relative humidity. For energy recovery from ventilating air, as described above, one can imagine that the moisture-adsorption capacity of a desiccant at higher relative humidities is more important.
Nevertheless, an energy recovery ventilator must function at all conditions it encounters and there are times, or seasons, when the outdoor air may be quite hot, but with a relatively low relative humidity. It is clear that one would prefer to have a desiccant with the characteristics of normal silica gel (Type 3) at some times and of modified silica gel (Type 5) at other times. Thus, it would be advantageous to provide an energy recovery unit which combines the moisture capacity characteristics of two or more different desiccants so as to maximize moisture transfer effectiveness from intake air to exhaust air.
According to one aspect of the invention, there is provided a regenerator energy exchange device, comprising:
a frame which is rotatable about a rotation axis; and
an energy exchange matrix supported by the frame and disposed to allow intake and exhaust air to pass therethrough in countercurrent flow. The matrix comprises a plurality of interchangeable segments, each segment containing at least one desiccant selected from the family of desiccants having Brunauer classification numbers between 1 and 5. The matrix contains desiccants of at least two different Brunauer classifications.
In one embodiment, each segment may contain a single class of desiccant thereon. Alternatively, at least one segment contains multiple different classes of desiccant thereon. In another embodiment, all segments contain multiple different classes of desiccant thereon. The segments may all contain the same multiple classes of desiccants, or they may all contain different multiple classes of desiccants.
In a preferred embodiment, the matrix contains at least one Type 5 desiccant. The matrix can additionally include at least one desiccant selected from the group consisting of Type 1 and Type 3 desiccants.
The frame includes a hub, a plurality of spokes extending from the hub, and a rim coupled to the spokes.
According to another aspect of the invention, there is provided a regenerator energy exchange device, comprising:
a frame which is rotatable about a rotation axis, the frame including a hub, a plurality of spokes extending from the hub, and a rim coupled to the plurality of spokes; and
an energy exchange matrix supported by the frame and disposed to allow intake and exhaust air to pass therethrough along a flow path. The matrix includes a plurality of zones disposed serially along the flow path and contains at least one desiccant selected from the family of desiccants having Brunauer classification numbers between 1 and 5. Each zone contains a desiccant of a different class.
In one embodiment, the matrix includes a first zone closest to an intake airstream inlet and a second zone closest to an exhaust airstream inlet.
In another embodiment, the matrix may comprise a plurality of interchangeable segments, each segment containing at least one of the desiccants.
The frame for the device may include a hub, a plurality of spokes extending from the hub, and a rim coupled to the plurality of spokes.
According to still another aspect of the invention, there is provided a regenerator energy exchange device, comprising a frame which is rotatable about a rotation axis, and an energy exchange matrix supported by the frame and disposed to allow intake and exhaust air to pass in counterflow therethrough along a flow path. The matrix contains at least two desiccants selected from the family of desiccants having Brunauer classification numbers between 1 and 5. At least one of the desiccants has a Brunauer classification number between 1 to 3 inclusive, and at least one of the desiccants has a Brunauer classification number between 3 and 5 inclusive. Each desiccant in the matrix has a different Brunauer classification number.
According to still another aspect of the invention, there is provided a regenerator energy exchange device, comprising a plurality of frames which are rotatable about a common rotation axis, and a corresponding plurality of energy exchange matrices, each matrix being supported by a frame and disposed to allow intake and exhaust air to pass in counterflow therethrough along a flow path. Each matrix contains at least one desiccant selected from the family of desiccants having Brunauer classification numbers between 1 and 5. The device includes a first matrix closest to an intake airstream inlet and a second matrix closest to an exhaust airstream inlet. In a preferred embodiment, the first matrix contains at least one desiccant having type 3 to type 5 Brunauer adsorption characteristics, and the second matrix contains at least one desiccant having type 1 to type 3 Brunauer adsorption characteristics.
These and other objects and advantages of the invention will in part be obvious and will in part appear hereinafter. The invention accordingly comprises the apparatus possessing the construction, combination of elements and arrangement of parts which are exemplified in the following detailed disclosure, the scope of which will be indicated in the claims.